1. Field of the Invention
The present invention relates to an IR radiation heating element which comprises a substrate and a coating film formed from a coating composition for IR radiation heating.
2. Description of the Prior Arts
Many coating film having various compositions are known and prepared as IR radiation heating coating film. When the coating composition for IR radiation heating is applied and dried to form a film on a hard surface, particularly a metal surface, it can radiate IR at high efficiency. One example of such coating composition for IR radiation heating comprises an inorganic binder consisting of alumina or silica and an oxide of a transition metal such as iron, cobalt, nickel, copper and titanium. The coating composition is applied on a surface of a heating element, dried and then sintered to form a coating having a thickness of several ten to several hundred .mu.m on the heater surface. Another example of the coating composition for IR radiation heating comprises water glass and an oxide, carbide or nitride of a metal such as zirconium, silicon, aluminum, iron, chromium, nickel, cobalt, titanium or manganese. This composition is coated on the surface of the heating element, dried at a temperature at which the water glass hardens and sintered to form a coating film having a thickness of several ten to several hundred .mu.m. The heating element having the coating film for IR radiation heating on its surface is widely used as heating means for industrial or domestic use.
The reason why the ceramic type coating film comprising the oxide, carbide or nitride of the metal is used for IR radiation heating is that its IR radiation characteristics is superior to the metal. FIG. 1 shows radiation characteristics of three kinds of surfaces, in which the curves 1 to 3 represent spectroradiation characteristic curves for the surfaces of following materials, respectively:
1: Stainless steel PA1 2: Coating film comprising Zr0.sub.2, Si0.sub.2 and Al.sub.2 O.sub.3 PA1 3: Coating film comprising a transition metal oxide (e.g. Fe.sub.2 O.sub.3, Mn.sub.2 O.sub.3 and Cr.sub.2 O.sub.3), ZrO.sub.2, SiO.sub.2 and Al.sub.2 O.sub.3
As understood from these characteristic curves, The stainless steel 1 has lower emissivity than the coating films 2 and 3 at all the wavelengths specified in FIG. 1. The coating 2 has higher emissivity in a longer wavelength range. The coating 2 has high emissivity at all wavelengths. From these results, it is apparent that the coatings 2 and 3 can give a more amount of radiation heat than the stainless steel when the same amount of heat is input to the heating element from a heat source.
However, the conventional ceramic type coating films for IR radiation heating still have some drawbacks. The first drawback is that design of the coating film is difficult since little is known about a relationship between the radiation characteristics and coating conditions such as a formulation of the composition and a thickness of the coating film. The second one is that since the coating film comprises the ceramics, it has poor resistance against mechanical shock so that it tends to be easily cracked or peeled off from the surface of the heating element when external stress such as flexure is applied thereon. Further, the ceramic coating tends to be cracked or peeled off by thermal shock if the substrate deforms thereby. The third one is that when the coating composition is applied and dried to form the film on the heating element, the coating composition comprising the inorganic binder has poor wettability with the substrate and is repelled so that it is difficult to form a uniform coating film. Thus, the surface of the heating element should be pretreated in some steps, which makes the coating conditions more complicate. In addition, the organic binder should be heat resistant to be used as a part of the heating element. The fourth one is that, when the heating element is used at a high temperature, for example, when it is red heated at a temperature of 800.degree. to 1,000.degree. C., the coating components, particularly the inorganic binder is thermally deteriorated and the adhesion of the coating film to the substrate is decreased.